1. Field of the Invention
This invention relates to a coil package and a bias tee package, and more particularly to a coil package having a high-frequency coil mounted therein, and a broadband bias tee package for supplying a high-frequency signal by superposing a DC component on the high-frequency signal.
2. Description of the Related Art
Recently, with the development of multi-media technology, there is an increasing demand for constructing optical communication networks that transmit high-speed, large-volume information at low costs over long distances. To meet the demand, there have been developed optical communication systems whose transmission rate is in the order of 10 Gb/s, and further, as high-speed, large-volume communication means, optical communication systems whose transmission rate is in the order of 40 Gb/s are under development.
On the other hand, an electronic circuit called a bias tee is used in optical transmitter-receivers, measurement equipment, and so forth. The bias tee is comprised of a coil and a capacitor, and supplies a high-frequency signal by superposing a DC component, e.g. a DC current or a DC voltage, on the high-frequency signal, without adversely affecting the high-frequency signal.
In the bias tee for use in optical communication at a transmission rate of 10 Gb/s or less (≦10 Gb/s), it is possible to use a small-sized surface-mount coil (surface-mount type having a size of approximately 1.0 mm×0.5 mm) that can be directly mounted on a printed circuit board, as the coil (inductor) forming the bias tee. If the bias tee is used in optical communication at a transmission rate of 10 Gb/s or less, there occurs no marked degradation in frequency characteristics even when such a coil is used.
However, in performing optical communication at a transmission rate in the order of 40 Gb/s beyond the order of 10 Gb/s, it is impossible to use a coil of the above-mentioned type, since a parasitic capacitance of the coil itself and earth capacitances thereof cannot be ignored and a self-resonance frequency thereof forms a stumbling block to the use in such an optical communication.
FIG. 17 is a diagram showing an equivalent circuit of a coil. The coil 100 includes not only its inherent inductance but also capacitors (parasitic capacitance or line capacitance) formed by wound electric wires per se, winding resistance, and so forth.
An equivalent circuit 100a of the coil 100 can be defined as a circuit in which a coil L0 and a resistance R0 are connected in series, and a part formed by series connection of L0 and R0 and a capacitor Cr are connected in parallel. Further, when lead wires of the coil 100 are mounted on a printed circuit board, earth capacitances appear at respective locations of pads (copper foils for soldering, for use in mounting a component on a printed circuit board), and therefore the equivalent circuit 100a looks as if it has capacitors C1 and C2 connected between the lead wires and ground GND.
The parasitic capacitance of the capacitor Cr assumes a very small value, and hence it raises no problem when the coil 100 is used with low frequency. However, when the coil 100 is used as a high-frequency circuit, the parasitic capacitance cannot be ignored.
This is because the impedance Z of the coil is equal to 2πfL (Z=2πfL), and hence as the frequency f becomes higher, the impedance Z becomes larger, whereas when the frequency f is equal to a certain frequency f0 (self-resonance frequency), the inherent inductance L of the coil and the parasitic capacitance of the capacitor Cr cause a resonance phenomenon, so that as the frequency f becomes higher, the parasitic capacitance becomes dominant to lower the impedance Z.
Therefore, in a frequency range higher than the self-resonance frequency f0 as a point of reverse curve, the impedance Z is lowered, so that the coil no longer serves as a desired inductor.
Further, f0=½π(LC)1/2 holds, so that as L and C are smaller, the self-resonance frequency f0 becomes higher, and the point of reverse curve causing a resonance phenomenon becomes higher. This widens a usable frequency range (the coil 100 can be used for higher frequency uses), whereas when C becomes larger (when the parasitic capacitance of the capacitor Cr and the earth capacitances of the capacitors C1 and C2 become larger), the self-resonance frequency f0 becomes lower, and the point of reverse curve causing a resonance phenomenon becomes smaller. This narrows the usable frequency range (the coil 100 cannot be used for high-frequency uses).
As described above, although the bias tee including the above-mentioned coil 100 can be used in optical communication where the transmission rate is in the order of 10 Gb/s, it cannot be used in optical communication at a transmission rate of 20 Gb/s or higher. To overcome this problem, in recent high-speed optical communication systems, a so-called conical coil which has a high self-resonance frequency has come into use.
FIG. 18 is a diagram showing the outline of the conical coil. The conical coil 110 has a conical shape and is formed by winding a conductor wire 111 covered with an insulating film, around an outer peripheral surface of a conical core 112 made of a resin material, such that the winding diameter of the conductor wire progressively decreases from one end to the other end of the coil (from the right end to the left end, as viewed in FIG. 18). Further, the opposite ends of the conductor wire 111 have the insulating film peeled off to expose copper wire 111a, for use as terminals.
FIG. 19 is a diagram showing an equivalent circuit of the conical coil 110. The equivalent circuit 110a of the conical coil 110 is configured such that coils L1 to Ln having different inductances are connected in series. In this case, the coils L1 to Ln of the conical coil 110 are sequentially arranged in series in the increasing order of the inductance, when viewed from the tip side of the conical shape.
The conical coil 110 is characterized in that it can ensure broadband characteristics of approximately several hundreds of KHz to several tens of GHz, and since the tip thereof has a small diameter, the value of inductance thereof is small and the parasitic capacitance thereof is held small, whereby it is possible to maintain its characteristics up to a high frequency of several tens of GHz.
It should be noted that the conical coil 110 has its highest frequency characteristics determined by the coil L1, and the frequency characteristics in a higher to a lower frequency bands are sequentially determined by the coil L1 to the coil Ln, respectively.
More specifically, the conical coil 110 is configured such that the high frequency characteristics are determined by the value of inductance of the coil L1, which is the first and smallest-diameter coil on the tip side of the conical coil 110 (the high frequency characteristics can be maintained by the coil L1 since the coil L1 has a small diameter and hence has a small inductance value), and the frequency characteristics of the conical coil 110 from a higher to a lower frequency hand are sequentially determined by the inductance values of the coils whose diameter increases from the coil L1 to the coil Ln.
A prior art bias tee formed by using the conical coil has been proposed e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2004-193886 (Paragraph numbers [0014] and [0019], and FIG. 1).
FIG. 20 is a diagram showing a manner of bonding the conical coil 110. The tip of the conical coil 110 is bonded (press-fitted) on a circuit board by heat or ultrasonic waves.
However, in general, the conical coil 110 is compact in size, i.e. approximately several mm long in the direction of the length thereof. Further, the conical coil 110 has an unstable shape, since the wire of winding thereof is thin, i.e. has a diameter of approximately several tens of μm. Therefore, the conical coil 110 is generally mounted in an IC package. Further, it is necessary to connect the conical coil 110 by accurate bonding manually performed by a skilled worker. Therefore, it can be used in limited areas or locations of devices, and is very difficult to handle.
Further, a lead wire can be extended from the tip of the conical coil 110 only by several hundreds of μm, and if it is further extended, the high frequency characteristics of the conical coil are degraded. Moreover, the characteristics of the conical coil 110 vary with the mounting angle thereof, and hence there is a problem that a large variation in the characteristics is caused when the conical coil 110 is mounted.
Furthermore, conventionally, the conical coil 110 cannot be subjected to reflow (reflow: to supply necessary amounts of solders having various shapes on a pattern of a printed circuit board, and collectively thermally fuse the solders by a heat source, to thereby metallically bond electronic components to the circuit board for electric conduction). Therefore, the conical coils 110 are mounted on circuit boards one by one, by bonding, which makes it impossible to expect improvement of productivity.